This invention relates to emergency evacuation equipment for aircraft, in particular to an inflation device for inflating an inflatable aircraft evacuation slide or other inflatable device.
The requirement for reliably evacuating airline passengers in the event of an emergency is well known. Emergencies at take-off and landing often demand swift removal of passengers from the aircraft because of the potential for injuries from fire, explosion, or sinking in water. A conventional method of quickly evacuating a large number of passengers from an aircraft is to provide multiple emergency exits, each of which is equipped with an inflatable evacuation slide.
Current state of the art emergency evacuation slide systems comprise an inflatable evacuation slide that is stored in a folded, uninflated state together with a source of inflation gas. The source of inflation gas typically comprises a gas generator, stored compressed gas, or a combination thereof. Pyrotechnic gas generators have an advantage in that they are small, lightweight, and produce a high volume of gas, however, the high temperature gas produced by a gas generator alone can cause numerous problems including sagging of the evacuation slide as the inflation gas cools and, in some cases melting or scorching of the fabric out of which the inflation slide is fabricated. Use of stored compressed gas by itself, although simple, implicates a weight penalty that must be paid for carrying a pressure vessel having sufficient capacity (in terms of volume and pressure) to inflate the evacuation slide over the wide operational temperature range specified for such slides. Additionally, where only a compressed gas is used to inflate the evacuation slide, a large drop in temperature occurs as the gases expand, often causing ice to form, which can block the flow of gas. These problems are further exacerbated by the extreme temperature range over which these inflation systems are required to operate, typically from −65° F. to +165° F.
Accordingly, state of the art emergency evacuation slide systems typically comprise a hybrid inflator, which utilizes a pyrotechnic gas generator in combination with a stored mixture of compressed and liquefied gases, typically a mixture of about 15% compressed Nitrogen or Argon together with about 85% Carbon Dioxide primarily in liquid form. The pyrotechnic gas generator is stored in a separate pressure vessel that is isolated from the stored gas by a frangible diaphragm. Upon initiation of the inflator, the pyrotechnic gas generator is ignited and pressure builds until the frangible diaphragm ruptures allowing the hot combustion products to flow from the gas generator compartment into the stored gas pressure vessel. The gas generator provides heat to vaporize the liquid Carbon Dioxide as well as providing additional gaseous combustion products. The liquid Carbon Dioxide absorbs heat from the gas generator so that the inflator output is reduced in temperature and therefore the inflatable device is less prone to sagging after inflation.
Use of a significant portion of liquefied gas provides great advantages in terms of the ratio of the volume of gas produced to the volume of stored liquid. Even better ratios could be obtained, however, if the percentage of liquefied Carbon Dioxide could be increased. Unfortunately, at very low temperatures, the vapor pressure of Carbon Dioxide is too low (less than 200 psia at −65° F.) to enable combustion of the pyrotechnic materials used in modern hybrid inflators. Accordingly, a minimum of about 15% Nitrogen, Argon or other gas that remains gaseous at −65° F. must be included so that a minimum pressure of about 1200 psi is maintained in the pressure vessel so that when the burst diaphragm between the gas generator and the stored gas pressure vessel ruptures, the sudden loss of pressure in the gas generator does not cause the pyrotechnic material to self-extinguish.
Accordingly, what is needed is a hybrid inflator that temporarily throttles the gas flow from the gas generator to the stored gas pressure vessel so that pressure within the gas generator is maintained irrespective of the pressure in the stored gas pressure vessel. Use of such a temporary throttle will enable a higher percentage of liquefied Carbon Dioxide to be used, resulting in a smaller pressure vessel with the concomitant reduction in weight and volume of the inflator system.